Specific tolerance to the hapten, p-azobenzenearsonate, can be induced in neonatal and adult mice when the hapten is presented in the form of a conjugate to the protein, human gamma globulin. The adult tolerant state is characterized by the preferential loss of certain clones of B-cells capable of producing antibody of a given idiotypic specificity. The kinetics of tolerance in these animals is of the same duration as that to the HCG carrier determinants themselves. Upon recovery from this tolerance, those clones capable of producing the given idiotypic specificity are delayed in their appearance. This preferential loss and delayed recovery of certain clones indicates a fine degree of control of immune responses heretofore not expected. The mechanisms responsible for this preferential loss and delayed recovery are to date not known but are under extensive investigation.